1. Field of Invention
The present invention relates to a light guide plate, surface light source device of side light type and liquid crystal display. More specifically, the invention relates to a light guide plate having a minor face to provide a side end face through which light is inputted and having a major face through which light is outputted, and to a surface light source device of side light type and liquid crystal display which employ the light guide plate.
2. Related Art
It has been known to apply a surface light source device of side light type to a device, for example, to a liquid crystal display for backlighting its liquid crystal display panel. This arrangement is suitable for giving the device a thin whole structure.
In general, a surface light source device of side light type employs a rod-shaped light source like cold cathode tube as a primary light source which is disposed beside a light guide plate (plate-shaped light guide). The primary light source emits light illumination light which is introduced into the light guide plate through a side end face (an incidence face) provided by a minor face of the light guide plate. The introduced illumination light propagates within the light guide plate, being emitted from a major face (an emission face) of the light guide plate on the way of propagation. The emitted light is supplied to an object to be illuminated such as liquid crystal display panel via an additionally arranged prism sheet, light diffusion sheet or the like.
Guide plates employed in surface light source devices of side light type are classified into two types depending on cross section shape, one having an approximately uniform thickness, the other having thickness tending to reduce according to distance from a side end face. In general, the latter gives a higher emission efficiency as compared with the former. A known light guide plate has a major face provided with a great number of projection rows running in a direction approximately perpendicular to an incidence face.
The projection rows provide a prismatic face which is capable of modifying emission directivity of the light guide plate in a plane perpendicular to the incidence end face. Usually, the prismatic face corrects emission directivity so that emitted light is gathered around an approximately frontal direction. Such directivity correction can be achieved by an alternative such as an additional prism sheet arrangement, which leads to increased number of components or more complicated structure.
To the contrary, the fore-mentioned method, according to which xe2x80x9ca great number of projection rows running in a direction approximately perpendicular to an incidence face are formed on a major facexe2x80x9d, is almost free from such a problem. However, this method is subject to another problem called xe2x80x9creflective appearancexe2x80x9d. This phenomenon, xe2x80x9creflective appearancexe2x80x9d, brings a local strong brightness mainly caused by emission in the vicinity of an incidence face, the emission contains light which comes via upper and lower edges of the incidence face or via right and left side edges.
Similar phenomena are apt to emerge in the vicinity of xe2x80x9cflank facesxe2x80x9d of a light guide plate. It is noted that xe2x80x9cflank facesxe2x80x9d of a light guide plate are minor faces (side end faces) adjacent to the incidence face at the right and left ends of the incidence face. In usual cases, such phenomena bring several excessively bright regions like bright lines.
Such abnormal emission deteriorates illumination quality of a surface light source device of side light type. And besides, if the surface light source device is applied to a liquid crystal display, display quality will be reduced.
An object of the present invention is to provide a light guide plate which is improved so that the above- described problems are solved. Another object of the present invention is to provide a surface light source device of side light type which is able to suppress abnormal emission by means of the improved light guide plate. A still another object of the present invention is to provide a liquid crystal display which is able to avoid display quality from falling by employing the surface light source device of side light type.
First, the present invention is applied to a light guide plate comprising minor faces to provide an incidence face for light input and flank faces, and major faces to provide an emission face for light output and a back face.
One of said major faces of the light guide plate in accordance with the present invention is provided with a great number of projection rows running in a direction approximately perpendicular to said incidence face, thereby providing a great number of valleys. According to a feature of the present invention, each of said valleys includes, at least in the vicinity of said incidence face, a curled-surface portion including a connected pair of approximately cylindrically surfaced slopes provided by a pair of projection rows located at both sides of each of said valleys.
It is preferable that said each valley includes, further in the vicinity of said incidence face, a curled-surface portion composed of a connected pair of approximately cylindrically surfaced slopes formed in foot portions of a pair of projection rows located at both sides of said each valley.
Said valleys each include in the vicinities of said flank faces, a curled-surface portion including a connected pair of approximately cylindrically surfaced slopes provided by a pair of projection rows located at both sides of said valleys.
Next, the present invention is applied to a surface light source device comprising a primary light source and a light guide plate which has minor faces to provide an incidence face and flank faces, and major faces to provide an emission face and back face, wherein said light guide plate is arranged to be supplied with light from said primary light source through said incidence face.
One of said major faces of the light guide plate employed in a surface light source device in accordance with the present invention is provided with a great number of projection rows running in a direction approximately perpendicular to said incidence face, thereby providing a great number of valleys.
And according to a feature of the present invention, each of said valleys includes, at least in the vicinity of said incidence face, a curled-surface portion including a connected pair of approximately cylindrically surfaced slopes provided by a pair of projection rows located at both sides of said valleys.
Each of said valleys includes, in the vicinities of said flank faces, a curled-surface portion including a connected pair of approximately cylindrically surfaced slopes provided by a pair of projection rows located at both sides each of said valleys.
Said great number of valleys are preferably configurated so that roundness tends to increase according to decreasing distance from said incidence face. In a manner like this, said great number of valleys are preferably configurated so that roundness tends to increase according to decreasing distance from said flank face (a relatively nearer one of two flank faces).
Further to this, a liquid crystal display in accordance with the present invention is provided by employing the above improved surface light source device in an illumination arrangement for illuminating a liquid crystal display panel.
In light guide plates or surface light source devices improved and featured according to the present invention, curled-surface configurations included in a great number of valleys give diverse directions to light which reaches there via an upper, lower, right or left edge of an incidence face, with the result that locally concentrated emission hardly occurs. And effect like this is expected with light which reaches there via an upper or lower edge of a flank face.
If curled-surface portions are formed in the vicinity of a flank face, the curled-surface configurations included in a great number of valleys give diverse directions to light which reaches there via an upper or lower edge the flank face, with the result that locally concentrated emission is avoided more surely.
It is further noted that gradually varying roundness of the valleys enables profile of valleys to avoid from changing sharply. This prevents local brightness unevenness, which would be caused if the profile had a sharp changing.
It is needless to say that the above-described features and advantages are inherited to liquid crystal displays in accordance with the present invention and bring an improved evenness of display brightness. More detailed description of the present invention is given hereafter with referring to the accompanied drawings.